The present invention relates to refrigeration lubricants having a partially fluorinated polymer backbone. More particularly, the present invention relates to refrigeration lubricants for use with tetrafluoroethane and preferably, 1,1,1,2-tetrafluoroethane (known in the art as R134a). R134a is a refrigerant which may replace dichlorodifluoromethane (known in the art as R12) in many applications because environmental concerns over the use of R12 exist.
R134a has been mentioned as a possible replacement for R12 because concern over potential depletion of the ozone layer exists. R12 is used in closed loop refrigeration systems; many of these systems are automotive air-conditioning systems. R134a has properties similar to those of R12 so that it is possible to substitute R134a for R12 with minimal changes in equipment being required. The symmetrical isomer of R134a is R134 (1,1,2,2-tetrafluoroethane); the isomer is also similar in properties and may also be used. Consequently, it should be understood that in the following discussion, "tetrafluoroethane" will refer to both R134 and R134a.
A unique problem arises in such a substitution. Refrigeration systems which use R-12 generally use mineral oils to lubricate the compressor; the present discussion does not apply to absorption refrigeration equipment. See for example the discussion in Chapter 32 of the 1980 ASHRAE Systems Handbook. R-12 is completely miscible with such oils throughout the entire range of refrigeration system temperatures which may range from about -45.6.degree. to 65.6.degree. C. Consequently, oil which dissolves in the refrigerant travels around the refrigeration loop and generally returns with the refrigerant to the compressor. The oil does not separate during condensation, although it may accumulate because low temperatures exist when the refrigerant is evaporated. At the same time, the oil which lubricates the compressor contains some refrigerant which may affect its lubricating property.
It is known in the industry that chlorodifluoromethane (known in the art as R22) and monochlorodifluoromethane/1-chloro-1,1,2,2,2-pentafluoroe thane (known in the art as R502) are not completely miscible in common refrigeration oils. See Downing, FLUOROCARBONS REFRIGERANT HANDBOOK, p. 13. A solution to this problem has been the use of alkylated benzene oils. Such oils are immiscible in R134a and are not useful therewith. This problem is most severe at low temperatures when a separated oil layer would have a very high viscosity. Problems of oil returning to the compressor would be severe.
R134a is not miscible with mineral oils; consequently, different lubricants will be required for use with R134a. However, as mentioned above, no changes to equipment should be necessary when the refrigerant substitution is made. If the lubricant separates from the refrigerant, it is expected that serious operating problems could result. For example, the compressor could be inadequately lubricated if refrigerant replaces the lubricant. Significant problems in other equipment also could result if a lubricant phase separates from the refrigerant during condensation, expansion, or evaporation. These problems are expected to be most serious in automotive air-conditioning systems because the compressors are not separately lubricated and a mixture of refrigerant and lubricant circulates throughout the entire system.
These problems have been recognized generally in the refrigeration art. Two recent publications by ASHRAE suggest that separation of lubricants and refrigerants presents problems, although no mention is made of R134a. These articles are Kruse et al., "Fundamentals of Lubrication in Refrigeration Systems and Heat Pumps," ASHRAE TRANSACTIONS 90(2B), 763 (1984) and Spauschus, "Evaluation of Lubricants for Refrigeration and Air-Conditioning Compressors," ibid, 784.
The following discussion will be more readily understood if the mutual solubility of refrigerants and various lubricating oils is considered in general with specific reference to R134a. Small amounts of lubricants may be soluble in R134a over a wide range of temperatures, but as the concentration of the lubricant increases, the temperature range over which complete miscibility occurs, i.e., only one liquid phase is present, narrows substantially. For any composition, two consolute temperatures, i.e., a lower and a higher temperature, may exist. That is, a relatively low temperature below which two distinct liquid phases are present and above which the two phases become miscible and a higher temperature at which the single phase disappears and two phases appear again may exist. A diagram of such a system for R502 refrigerant is shown as FIG. 2 in the Kruse et al. paper mentioned above. A range of temperatures where one phase is present exists and while it would be desirable that a refrigeration system operate within such a range, it has been found that for typical compositions, the miscible range of lubricants with R134a is not wide enough to encompass the typical refrigeration temperatures.
Disclosures which are concerned with the choice of lubricants when R134a is used as a refrigerant exist. Polyalkylene glycols were suggested to be used in Research Disclosure 17483, October 1978 by DuPont. Specific reference was made to such oils produced by Union Carbide Corporation under the trade names "ULCON" (sic) LB-165 and UCON 525. It is stated that these oils are miscible in all proportions with R134a at temperatures at least as low as -50.degree. C. It is believed that "ULCON" (sic) LB-165 and UCON 525 are polyoxypropylene glycols which have a hydroxy group at one end of each molecule and a n-butyl group at the other end.
The use of synthetic oils for refrigeration systems including polyoxyalkylene glycols is discussed by Sanvordenker et al. in a paper given at an ASHRAE Symposium, June 29, 1972. The authors make the point that polyglycols should properly be called ethers and esters rather than glycols because the terminal hydroxyl groups are bound by ester or ether groups. It is stated that this substitution makes them suitable for lubrication.
U.S. Pat. No. 4,428,854 discloses the use of R134a as an absorption refrigerant where organic solvents are used as absorbing agents. An example is tetraethylene glycol dimethyl ether. A related patent U.S. Pat. No. 4,454,052 also discloses polyethylene glycol methyl ether used as an absorbent along with certain stabilizing materials for refrigerants such as 134a.
Japanese Patent Publication 96684 dated May 30, 1985 addresses the stability problems of refrigerants. The reference teaches that perfluoro ether oligomers are one class of useful lubrication oils.
U.S. Pat. No. 4,267,064 also recommends the use of polyglycol oils, particularly for rotary compressors. It is indicated that viscosities in the range of 25-50 centistokes (CS) at 98.9.degree. C. are needed plus a viscosity index greater than 150. Many refrigerants are mentioned but not tetrafluoroethane.
Japanese published application No. 51795 of 1982 relates to antioxidants and corrosion inhibitors for use with various polyether type synthetic oils. The tests were carried out with R-12, which does not exhibit the immiscible character of R134a.
Japanese published patent application 96,684 published May 30, 1985 addresses the stability problems of refrigerants. The reference mentions 12 refrigerants including tetrafluoroethane. The reference also teaches six classes of lubricants including perfluoro ether oligomer, fluorinated silicone, fluorinated oxethane, chlorotrifluoro ethylene polymer, fluorinated polyphenyl ether, and perfluoroamine.
U.S. Pat. No. 4,431,557 relates to additives used in synthetic oils. Many refrigerants are mentioned, but not tetrafluoroethane, and the patentees gave no indication of concern for miscibility of the refrigerants and the lubricants.
Commonly assigned U.S. Pat. No. 4,755,316 teaches a compression refrigeration composition. The refrigerant is tetrafluoroethane while the lubricant is at least one polyoxyalkylene glycol which is at least difunctional with respect to hydroxyl groups, has a molecular weight between 300 and 2,000, has a viscosity of about 25-150 centistokes at 37.degree. C., has a viscosity index of at least 20, and is miscible in combination with the tetrafluoroethane in the range between -40.degree. C. and at least +20.degree. C. The reference does not teach or suggest the present refrigeration compositions. See also U.S. Pat. No. 4,948,525.
U.K. Patent 1,087,283; U.S. Pat. Nos. 3,483,129; 4,052,277; 4,118,398; 4,379,768; 4,443,349; 4,675,452; 4,827,042; 4,898,991; and 4,931,199; International Publications WO 87/02992 and WO 87/02993; and Kokai Patent Publication 118,598 published May 11, 1989 teach perfluorinated ethers and perfluoropolyethers as lubricants. The references do not teach that their lubricants are useful with R134a. Also, Kokai Patent Publication 146,996, published June 30, 1987, teaches the addition of a perfluoroalkylpolyether as an extreme pressure additive to mineral oil.
Carre, "The Performance of Perfluoropolyalkyether Oils under Boundary Lubrication Conditions", TRIBOLOGY TRANSACTIONS 31(4), 437 (1987) and Carre, 1988 Air Force Report discuss the problems of perfluoropolyalkylethers and boundary lubrication in spacecraft.
U.K. Patent 1,354,138 teaches compounds of the formula: EQU R--(--[(L)(CH).sub.z --CH.sub.2 --O--].sub.x --R.sub.f).sub.m
wherein L is --H or --CH.sub.3 and z is 0, 1, or 2 on page 1, lines 9-41. As such, the oxyalkylene group can be oxymethylene when z is 0, ethylene oxide when z is 1 and L is --H, straight chain propylene oxide when z is 2 and L is --H, branched propylene oxide when z is 1 and L is --CH.sub.3, and branched oxypentylene when z is 2 and L is --CH.sub.3. These materials are taught to be useful as surfactants.
U.S. Pat. No. 4,079,084 teaches a compound having a chain of repeating units which may be oxyalkylidine, oxymethylene, oxyalkylene, imino alkylene, or secondary amido chains and at least two terminal perfluorocarbon groups of at least three carbon atoms. For the oxyalkylene unit, the reference teaches ethylene oxide, propylene oxide, or butylene oxide. These materials are taught to be useful as surfactants.
U.S. Pat. No. 2,723,999 teaches compounds of polyethylene glycols or polypropylene glycols. These materials are taught to be useful as surface active agents.
U.S. Pat. No. 4,359,394 teaches that a minor portion of an additive such as a fluorinated aromatic, for example, benzotrifluoride, can be added to a conventional lubricant such as mineral oil. The reference does not teach that a fluorinated aromatic alone is useful as a lubricant.
U.S. Pat. No. 4,944,890 teaches a refrigerant composition of R134a and a copolymer of a fluorinated olefin and nC.sub.4 H.sub.9 OCH.dbd.CH.sub.2.
Because it is expected that R134a will become widely used in the field of refrigeration and air-conditioning, new improved lubricants useful with R134a are needed in the art.